Optical waveguide circuit and manufacturing method thereof

ABSTRACT

An optical waveguide circuit comprising a plurality of first cores ( 203 ) arranged at intervals widening as they are away from the branch point or the joining point of optical signal, a clad ( 205 ) filling at least these first cores, and second cores ( 204 ) provided between the first cores and the clad and formed in the gap between the first cores in the vicinity of the branch point or the joining point while covering the first cores at least partially. Refractive index of the second core is larger than that of the clad, the boundary between the second core and the clad is smooth and the film thickness of the second core formed in the gap between the first cores is decreased as the interval of the plurality of first cores widens.

TECHNICAL FIELD

The present invention relates to an optical waveguide circuit used foroptical communications and a manufacturing method thereof andparticularly, to an optical waveguide circuit having a branch portionand a manufacturing method thereof.

BACKGROUND ART

An optical waveguide circuit, especially, a Planar Lightwave Circuit(PLC) in which an optical waveguide was formed on a planar surface hasbeen extensively used as a key device for supporting a recent opticalcommunication network system. In particular, passive device such as anoptical multi/demultiplexer or an optical branch device using a silicaoptical waveguide has become indispensable for practical application ofa low-priced and high-performance system in the fields from a backbonenetwork represented by a large-capacity optical communication to anaccess-based network and have already been put into practical use andcommercial mass production.

As an example of the PLC, an Arrayed Waveguide Grating (AWG) is shown inFIGS. 27 and 28. FIG. 27 is a top view of the AWG device; and FIG. 28 isa cross-sectional view taken along the line VIIIb—VIIIb in FIG. 27. Thisdevice has, at the inside of cladding layers thereof constituted bylower cladding 802 and upper cladding 804 formed on substrate 801, awaveguide 811 that propagates a wavelength-multiplexed optical signal, afirst slab waveguide 812 connected to the waveguide 811, a waveguide 815that separately propagates optical signals of different wavelengths, asecond slab waveguide 814 connected to the waveguide 815, and arrayedwaveguides 813 that connect the first slab waveguide 812 and second slabwaveguide 815. The device has a function of demultiplexing thewavelength-multiplexed optical signal into different wavelengths, orcontrary, multiplexing optical signals of different wavelengths onto oneoptical fiber. In this configuration, as shown in FIG. 28, the upperportions of cores 803 and gaps between the cores 803 in the arrayedwaveguides 813 are covered by the upper cladding 804.

An operational principle of the device will be briefly described belowwith the case of demultiplex taken as an example.

A wavelength-multiplexed optical signal incident into the waveguide 811is scattered by diffraction in the first slab waveguide 812 and entersthe arrayed waveguides 813 having a plurality of cores 803. Sinceoptical path-length differences are provided between adjacent waveguidesof the arrayed waveguides 813, the tilt of a wave front that ispropagated through the arrayed waveguide differs depending on thewavelength. The optical signals emitted from the arrayed waveguides 813are directed into the second slab waveguide 814, where the opticalsignal is collected for each output channel according to the tilt. Theeach collected optical signal is then wavelength demultiplexed, andoutput from the waveguide 815.

As another example of the PLC, a coupler is shown in FIGS. 29 and 30.FIG. 29 is a top view of the coupler; and FIG. 30 is a cross-sectionalview taken along the line IXb—IXb in FIG. 29. The coupler has aconfiguration in which two waveguides (cores 903) are nearby arranged toeach other in proximity waveguides area 912 having length L and iswidely used as an optical communication device such as light branching,light converging, wavelength filter, or optical switch using thermoopticeffect. In the case of light branching, an optical signal entering frominput waveguide 911 in FIG. 29 and the adjacent waveguide interfere witheach other in the proximity waveguides area 912 having the couplinglength L to allow the optical signal to diverge into two. The resultantoptical signals are then output from output waveguides 913A and 913B,respectively. The splitting ratio in this case can be changed dependingon the length L.

Also in this configuration, as shown in FIG. 30, the upper portions ofthe two cores 903 formed on the lower cladding 902 on the substrate 901and the gap between the adjacent cores are covered by the upper cladding904 in the proximity waveguides area 912.

Currently, there is request to develop an optical waveguide circuit suchas the abovementioned AWG or coupler having low insertion loss andhaving a reduced size. For example, the insertion loss in the PLC deviceis required to be minimized as much as possible for convenience ofsystem design. At the same time, the size of the device is required tobe reduced as much as possible for cost reduction in manufacturing ofthe device or integration of functions.

In particular, reduction of propagation loss is a common subject in thereduction of the insertion loss in the PLC device. One of the majorfactors of the propagation loss in the PLC is uneven shape of a boundarysurface between the core and cladding, that is, scattering loss due tosurface roughness. FIGS. 31, 32 and 33 schematically show the roughnesson a boundary surface between core 1003 and upper cladding 1004. FIG. 31is a top view showing roughness on the core-side surface of the PLCdevice; FIG. 32 is a cross-sectional view taken along the line Xb—Xb inFIG. 31; and FIG. 33 is a cross-sectional view taken along the lineXc—Xc of FIG. 31. The surface roughness on the core 1003 formed on lowercladding 1002 on substrate 1001 is caused by film surface roughness thathas occurred at the time of coating of core layer or pattern roughnessdue to photolithography and etching at the time of patterning of thecore.

In order to reduce the size of the PLC device, it is effective toincrease the core-cladding refractive index difference Δ and to decreasethe minimum curvature radius of the waveguide. However, in particular,the more the core-cladding refractive index difference Δ is increased,the more the scattering loss tends to be increased. Therefore, when thecore-cladding refractive index difference Δ is increased forminiaturization of the device, the core surface must be smoothed forsuppressing the scattering loss.

Further, radiation loss arising at a branch point (diverging point) is amajor problem particularly in the AWG. The radiation loss at a branchpoint of the AWG, that is, at a coupling portion between the slab andarray accounts for approximately half of the insertion loss in theentire AWG. In order to reduce the radiation loss at a branch portion,it is effective to reduce the distance between the split cores that havebranched at a branch point. However, limitation of the accuracy in thephotolithography or etching process forces the split cores to be spacedat least about 1 μm apart in general. As shown in FIG. 28, the claddingmaterial is filled in between the cores 803 in general and the boundarybetween the core 803 and cladding 804 is well-defined. Therefore, mostof the signal light that enters the gaps between the sprit cores afterpropagating through the slab waveguide 812 is introduced into thecladding, which causes the radiation loss. The same can be said for thecase where the signal lights enter the slab waveguide 814 from arrayedwaveguides 813.

To cope with the problem of the radiation loss arising in the AWG, apublication of patent applications (JP 2000-147283A) has disclosed aconfiguration in which, as shown in FIGS. 35 and 36 that showcross-sectional views taken along the lines XIb—XIb and XIc—XIc in FIG.34 respectively, buried layer 1101 having a refractive index higher thanthe refractive index of claddings 802 and 804 and lower than that ofcores 803 is formed between the cores 803 and the thickness of theburied layer 1101 becomes thinner as the distance between the coresbecomes wider. With this configuration, the electromagnetic fielddistribution between the cores 803 at the coupling portion between theslab waveguide 812 and arrayed waveguides 803 is gradually changed toreduce the radiation loss at the branch point. However, in thisconfiguration, the shape greatly depends upon etching condition andconsequently, manufacture of a device is difficult, which may result invariation of the shape in a wafer surface or between wafers. Note that,first slab waveguide 812, arrayed waveguides 813, and second slabwaveguide 814 are formed between input waveguide 811 and outputwaveguide 815. Lower cladding 802 is formed on substrate 801.

On the other hand, a problem lies in that the coupling length L of thedirectional coupler becomes longer especially when the refractive indexdifference Δ between the core 903 and claddings 902, 904 is increased.That is, although it is effective to increase Δ and decrease the minimumcurvature radius of the waveguide for the miniaturization of the device,the increase of Δ strengthens the state where the signal light isconfined in the core. Accordingly, the interference to the proximitywaveguide is reduced, with the result that it becomes necessary toincrease the coupling length L for obtaining a desired splitting ratio.It is possible to reduce the coupling length by narrowing the distancebetween the proximity waveguides (that is, distance between the cores903 in the proximity waveguides area 912). However, the distance betweenthe waveguides is restricted by the accuracy in the photolithography oretching process, so that the coupling length needs to be increased.

The present invention has been made to solve the above problems and anobject thereof is to reduce loss in the optical waveguide circuit, andto reduce the device size as well as to increase the degree ofintegration.

DISCLOSURE OF THE INVENTION

To achieve the above object, according to a first aspect of the presentinvention, there is provided an optical waveguide circuit including: afirst core; a cladding that buries the first core; and a second corethat is formed between the first core and cladding and covers at least apart of the first core, wherein the refractive index of the second coreis higher than the refractive index of the cladding, and the boundarybetween the second core and cladding is made smooth. In the presentinvention, by making the boundary between the second core and claddingsmooth, propagation loss in the optical waveguide can be reduced.

In the optical waveguide circuit according to the present invention,when the first core that is covered by the second core has asubstantially rectangular cross-section, the second core covers, forexample, the upper surface and both side surfaces of the first core.

It is possible to set the thickness of the second core at a value lessthan or equal to twice the thickness of the first core.

Further, the second core having a refractive index higher than therefractive index of the cladding constitutes the core of the opticalwaveguide together with the first core. Therefore, it is preferable thatthe refractive index of the second core be near that of the first core.For example, it is possible to set the refractive index of the secondcore at a value less than or equal to 1.01 times that of the first core.

According to a second aspect of the present invention, there is providedan optical waveguide circuit that allows an optical signal propagatingthrough at least one optical waveguide to branch into a plurality ofoptical waveguides, or converges optical signals propagating through aplurality of waveguides into at least one optical waveguide, theplurality of optical waveguides including: a plurality of first coreseach interval of which becomes wider as the first cores get away from abranch point or converging point of an optical signal; a cladding thatburies at least the first cores; a second core that is so formed betweenthe first cores and cladding as to cover up at least a part of each ofthe first cores and is formed in the gaps between the first cores at theposition in the vicinity of the branch point or converging point,wherein the refractive index of the second core is higher than therefractive index of the cladding, the boundary between the second coreand cladding is made smooth, and the film thickness of the second coreformed in the gaps between the first cores becomes thinner as theinterval between the first cores becomes wider.

The optical waveguide circuit described above can be used as a branchcircuit that allows an optical signal to branch into a plurality ofoptical waveguides, or as a converging circuit that converges aplurality of waveguides. The above optical waveguide circuit can beconfigured as a Y-shaped branch circuit.

In the present invention, the second core whose film thickness graduallybecomes thinner as each interval between the first cores becomes wideris provided between the first cores in the vicinity of the branch pointor converging point of an optical signal. As a result, it is possible toobtain an advantage corresponding to that obtained by reducing theinterval between the cores that branch at the branch point as much aspossible. Further, since the boundary between the second core andcladding is made smooth, the radiation loss at the branch point andtransmission loss in the optical waveguide can be reduced.

According to a third aspect of the present invention, there is providedan optical waveguide circuit including: a first slab waveguide connectedat least one input waveguide; a second slab waveguide connected at leastone output waveguide; and arrayed waveguides formed between the firstand second waveguides with optical path length differences, the arrayedwaveguides including: a plurality of first cores; a cladding that buriesthe first cores; a second core that is so formed between the first coresand cladding as to cover up at least a part of each of the first coresand that is formed in the gaps between the first cores at least atconnection areas between the first and second slab waveguides and thearrayed waveguides and areas near the connection areas, wherein therefractive index of the second core is higher than the refractive indexof the cladding, the boundary between the second core and cladding ismade smooth, and the film thickness of the second core formed in thegaps between the first cores of the arrayed waveguides becomes thinneras each interval between the first cores becomes wider.

The above second core prevents the signal light that propagates throughthe first slab waveguide and enters the gaps between the first coresfrom being radiated into the cladding, thereby reducing the radiationloss.

According to a fourth aspect of the present invention, there is providedan optical waveguide comprising proximity waveguides in which aplurality of first cores are nearby arranged to each other, theproximity waveguides including: a plurality of first cores; a claddingthat buries the first cores; and a second core that is formed betweenthe first cores and cladding to cover up at least a part of each of thefirst cores and is formed in the gaps between the first cores, whereinthe refractive index of the second core is higher than the refractiveindex of the cladding, and the boundary between the second core andcladding is made smooth.

The above optical waveguide circuit can be used as a coupler includingat least two proximity waveguides. By providing the second core, evenwhen the interval between the first cores of the proximity waveguides isnot changed from the conventional optical waveguide circuit, it ispossible to obtain an advantage corresponding to that obtained byreducing an interval between the proximity waveguides, so that thecoupling length of the coupler can be shortened.

According to a fifth aspect of the present invention, there is provideda manufacturing method of an optical waveguide circuit, including atleast the steps of: forming a core layer; selectively etching the corelayer to form a first core; forming a second core layer that covers theupper surface and both side surfaces of the first core, the second corebeing made of a material having a refractive index higher than therefractive index of the cladding; applying a heat reflow to the secondcore layer to smooth the surface thereof to complete a second core; andforming the cladding on the second core.

According to a sixth aspect of the present invention, there is provideda manufacturing method of an optical waveguide circuit that allows anoptical signal propagating through at least one optical waveguide tobranch into a plurality of optical waveguides, or converges opticalsignals propagating through a plurality of optical waveguides into atleast one optical waveguide, the method including at least the steps of:forming a core layer; selectively etching the core layer to form aplurality of first cores each interval of which becomes wider as thefirst cores get away from a branch point or converging point of anoptical signal; forming a second core layer on the upper portion of eachof the first cores and between the first cores at least at the areaincluding the portion near the branch point or converging point of thefirst cores, the second core being made of a material having arefractive index higher than the refractive index of the cladding;applying a heat reflow to the second core layer to smooth the surfacethereof and forming a second core such that the film thickness of thesecond core layer that is formed in the gaps between the first coresbecomes thinner as the interval between the first cores becomes wider;and forming the cladding on the second core.

According to a seventh aspect of the present invention, there isprovided a manufacturing method of an optical waveguide circuitincluding: a first slab waveguide connected at least one inputwaveguide; a second slab waveguide connected at least one outputwaveguide; and arrayed waveguides including a plurality of cores, and isformed between the first and second slab waveguides with optical pathlength differences, the method including at least the steps of: forminga core layer; selectively etching the core layer to form the pluralityof first cores each interval of which becomes wider as the first coresget away from a connection point between the first and second slabwaveguides; forming a second core layer on the upper portions of each ofthe first cores and between the first cores at least at the areaincluding connection areas between the first and second slab waveguidesand the first cores and the portion near the connection areas, thesecond core being made of a material having a refractive index higherthan the refractive index of the cladding; applying a heat reflow to thesecond core layer to smooth the surface thereof and forming a secondcore such that the film thickness of the second core layer that isformed in the gaps between the first cores becomes thinner as theinterval between the first cores becomes wider; and forming the claddingon the second core.

According to an eighth aspect of the present invention, there isprovided a manufacturing method of an optical waveguide circuitincluding proximity waveguides in which a plurality of first cores arenearby arranged to each other, the method including at least the stepsof: forming a core layer: selectively etching the core layer to form theplurality of first cores; forming a second core layer on the upperportion of each of the first cores and between the first cores at leastthe area including the proximity waveguides and the portion near theproximity waveguides, the second core layer being made of a materialhaving a refractive index higher than the refractive index of thecladding; applying a heat reflow to the second core layer to smooth thesurface thereof to obtain a second core; and forming the cladding on thesecond core.

According to the present invention, roughness in the vicinity of theboundary between the core and cladding can be smoothed, thereby reducingthe propagation loss in the waveguide.

The present invention can gradually reduce the equivalent refractiveindex between the cores as the interval between the cores becomes wider.Further, it is possible to obtain a cross-section of the core smootherthan in the conventional optical waveguide circuit. As a result, thescattering loss at the portion near the branch point in the branchcircuit or converging circuit can be reduced.

Further, according to the present invention, it is possible to increaseeffusion of a propagating light into between the cores in proximitywaveguide area, so that the coupling length of the coupler or the likecan be shortened..

The present invention can be used to reduce loss in a Y-shaped branchcircuit or arrayed waveguide grating.

With the manufacturing method of the optical waveguide circuit accordingto the present invention, the optical waveguide circuit can bemanufactured with a high yield.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing an optical waveguide according to a firstembodiment of the present invention;

FIG. 2 is a cross-sectional view taken along the line Ib—Ib in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line Ic—Ic in FIG. 1;

FIG. 4 is a top view showing an optical waveguide circuit according to asecond embodiment of the present invention;

FIG. 5 is a cross-sectional view taken along the line IIb—IIb in FIG. 4;

FIG. 6 is a cross-sectional view taken along the line IIc—IIc in FIG. 4;

FIG. 7 is a cross-sectional view taken along the line IId—IId in FIG. 4;

FIG. 8 is a top view showing an optical waveguide circuit according to athird embodiment of the present invention;

FIG. 9 is a cross-sectional view taken along the line IIIb—IIIb in FIG.8;

FIG. 10 is a cross-sectional view taken along the line IIIc—IIIc in FIG.8;

FIG. 11 is a cross-sectional view taken along the line IIId—IIId in FIG.8;

FIG. 12 is a top view showing an optical waveguide circuit according toa fourth embodiment of the present invention;

FIG. 13 is a cross-sectional view taken along the line IVb—IVb in FIG.12;

FIG. 14 is a cross-sectional view taken along the line IVc—IVc in FIG.12;

FIG. 15 is a cross-sectional view taken along the line IVd—IVd in FIG.12;

FIG. 16 is a top view showing an optical waveguide circuit according toa fifth embodiment of the present invention;

FIG. 17 is a cross-sectional view taken along the line Vb—Vb in FIG. 16;

FIG. 18 is a cross-sectional view taken along the line Vc—Vc in FIG. 17;

FIG. 19 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to a sixth embodiment of the presentinvention in a step order;

FIG. 20 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 21 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 22 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 23 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 24 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 25 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 26 is a cross-sectional view showing a method of manufacturing anoptical waveguide circuit according to the sixth embodiment of thepresent invention in a step order;

FIG. 27 is a top view showing a conventional optical waveguide circuit;

FIG. 28 is a cross-sectional view taken along the line VIIIb—VIIIb inFIG. 27;

FIG. 29 is a top view showing a conventional optical waveguide circuit;

FIG. 30 is a cross-sectional view taken along the line IXb—IXb in FIG.29;

FIG. 31 is a top view showing a conventional optical waveguide circuit;

FIG. 32 is a cross-sectional view taken along the line Xb—Xb in FIG. 31;

FIG. 33 is a cross-sectional view taken along the line Xc—Xc in FIG. 31;

FIG. 34 is a top view showing a conventional optical waveguide circuit;

FIG. 3 is a cross-sectional view taken along the line XIb—XIb in FIG.34; and

FIG. 36 is a cross-sectional view taken along the line XIc—XIc in FIG.34.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the accompanying drawings.

[First Embodiment]

An optical waveguide according to a first embodiment of the presentinvention is shown in FIGS. 1, 2 and 3. FIG. 1 is a top view; FIG. 2 isa cross-sectional view taken along the line Ib—Ib in FIG. 1; and FIG. 3is a cross-sectional view taken along the line Ic—Ic in FIG. 1.

The optical waveguide includes lower cladding 102 formed on substrate101, first core 103 formed on the lower cladding 102 and having asubstantially rectangular cross-section, second core 104 that covers theupper surface and both side surfaces of the first core 103, uppercladding 105 so formed on the lower cladding 102 as to cover up thefirst and second cores 103 and 104.

The surface of the second core 104 is made smooth. Roughness occurs onthe surface of the first cladding 103 at its formation time. However, byproviding the second core 104 between the first core 103 and uppercladding 105 and by making the boundary surface between the second core104 and upper cladding 105 more smooth than that between the second core104 and first core 103, it is possible to smooth the surface roughnessof the first core 103 in terms of results, thereby reducing thescattering loss due to roughness on the core surface. Accordingly, evenwhen the core-cladding refractive index difference Δ is increased forthe miniaturization of the device, it is possible to reduce an increasein the scattering loss due to the increase in the core-claddingrefractive index difference Δ. Therefore, the reduction in device sizecan be realized while reducing an increase in the waveguide propagationloss.

The refractive index of the second core 104 needs to be higher than thatof the upper cladding 105. However, when the refractive index of thesecond core 104 is too high, a change in a propagation constant causedby the second core 104 becomes large. Therefore, it is preferable thatthe refractive index of the second core 104 be near that of the firstcore 103. More concretely, it is desirable that the refractive index ofthe second core 104 be less than or equal to 1.01 times that of thefirst core 103.

The film thickness of the second core 104 should be larger than thedepth of the roughness on the upper or side surface of the core 103.However, when the film thickness of the second core 104 is too large, achange in a propagation constant caused by the second core 104 becomeslarge. Therefore, it is preferable that the film thickness of the secondcore 104 be less than or equal to 2 times that of the core 103.

A concrete example of the first embodiment will be described below. Asilicon substrate is used as the substrate 101. A silica-based materialprepared by doping boron and phosphorus (BPSG) is used as the materialsof the lower and upper claddings 102 and 105. The film thicknesses ofeach of the lower and upper claddings 102 and 105 is set to 10 μm, andthe refractive index thereof is set to 1.450. Silicon oxynitride (SiON)is used as the material of the first core 103. Both the thickness andwidth of the first core 103 are set to 3 μm. The refractive indexthereof is set to 1.480. As a result, the refractive index difference Δbetween the first core 103 and the claddings 102, 105 is set to 2%. BPSGis used as the material of the second core 104. The film thickness ofthe second core 104 is set to 0.5 μm, and the refractive index thereofis set to 1.480, which is the same value as that of the first core 103.

A heat reflow is applied to smooth the surface of the second core 104.The reason of selecting the BPSG as the material of the second core 104is that the softening temperature thereof is lower than that of thefirst core 103. That is, it is necessary for the material of the secondcore 104 to have the softening temperature lower than that of the firstcore 103 in order to smooth the surface of the second core 104 withoutdeforming the shape of the first core 103 at the time of the heatreflow.

With the above configuration, the depth of the surface roughness of thesecond core 104 can be reduced to 10 nm or less although the depth ofthe surface roughness of the first core 103 is about 100 nm in the casewhere the second core 104 is not provided. As a result, the waveguidepropagation loss is reduced from 0.2 dB/cm, which is obtained in thecase where the second core is not provided, to 0.04 dB/cm.

In the first embodiment, the second core 104 covers the upper surfaceand both side surfaces of the first core 103. However, the second core104 may cover at least a part of the first core 103. The second core 104may further cover the lower surface of the first core 103 in addition tothe upper and both side surfaces thereof.

[Second Embodiment]

An optical waveguide circuit according to a second embodiment of thepresent invention is shown in FIGS. 4, 5, 6 and 7. FIG. 4 is a top view;FIG. 5 is a cross-sectional view taken along the line IIb—IIb in FIG. 4;FIG. 6 is a cross-sectional view taken along the line IIc—IIc in FIG. 4;and FIG. 7 is a cross sectional view taken along the line IId—IId inFIG. 4.

The optical waveguide circuit allows an optical signal to branch fromone or more optical waveguides 211 to a plurality of optical waveguides213, or allows a plurality of optical signals to converge from aplurality of optical waveguides 213 to one or more optical waveguides211. That is, the optical waveguide circuit can be used as a branchcircuit that allows an optical signal to branch into a plurality ofoptical waveguides 213, or as a converging circuit that converges aplurality of optical waveguides 213. Note that the optical waveguidecircuit can be used as a branch circuit or converging circuit for anoptical signal having a single-wavelength or a plurality of opticalsignals having different wavelengths.

The plurality of optical waveguides 213 include lower cladding 202formed on substrate 201, a plurality of first cores 203 formed on thelower cladding 202 such that each interval between first cores 203becomes wider as the first cores 203 get away from a connection areabetween the plurality of optical waveguides 213 and slab waveguide 212which is the branch point or converging point of an optical signal,second core 204 that covers the upper and both side surfaces of each ofthe first cores 203 and that is formed in a gap between each adjacentpair of first cores 203 at the portion near the connection area betweenthe slab waveguide 212 and the plurality of optical waveguides 213, andupper cladding 205 so formed on the lower cladding 202 as to bury thefirst and second cores 203 and 204.

As in the case of the first embodiment, the refractive index of thesecond core 204 is higher than those of the claddings 202 and 205, andthe boundary between the second core 204 and cladding 205 is madesmooth.

The second core 204 is formed from the upper portion of a first core203S of the slab waveguide 212 to the upper portions of the first cores203 of the optical waveguides 213. The film thickness of the second core204 formed between the first cores 203 becomes thinner as an intervalbetween each adjacent pair of first cores 203 becomes wider. By formingthe second core 204 as described above, a configuration in which aninterval between each adjacent pair of the cores smoothly increases from0 is got. The configuration as described above can be obtained by, forexample, an application of the heat reflow process after film formationof the second core 204. The above configuration may be realized by anapplication of a spin-coating of a liquid material such as polymer resinor spin-on-glass resin. This configuration gradually reduces theequivalent refractive index between the cores in the waveguides 212 and213 and consequently, prevents radiation from occurring at the branchpoint or converging point. Further, as compared to the conventionalexample of the optical waveguide circuit shown in FIGS. 34 to 36, it canbe seen that the second core 204 is formed also on the upper portions ofthe first cores 203 and 203S, and the boundary surface between thesecond core 204 and upper cladding 205 has a smooth curved surface.Therefore, an advantage of a low Polarization Dependant Loss (PDL) canbe obtained.

As described above, in the second embodiment, the second core 204 whosefilm thickness gradually becomes thinner as an interval between theadjacent pair of the first cores becomes wider is provided between thefirst cores 203 in the vicinity of the branch point or converging pointof an optical signal. As a result, it is possible to obtain an advantageequivalent to that obtained by reducing an interval between the coresthat branch at the branch point as much as possible. Further, since theboundary between the second core 204 and the cladding is made smooth,the radiation loss at the branch point and transmission loss in theoptical waveguide can be reduced.

A concrete example of the second embodiment will be described below. Theoptical waveguide circuit is configured as a star-shaped branch circuithaving 1-input and 8-outputs. The material of the substrate, thematerial and size of each waveguide layers, the refractive index and thelike are the same as those in the first embodiment. Each intervalbetween the first cores 203 at the connection point between theplurality of optical waveguides 213 and slab waveguide 212 is set to 1μm. The interval between the first cores 204 is allowed to spread up to20 μm with the assumption that the propagation length z from aconnection point between the slab waveguide 212 and the plurality ofoptical waveguides 213 is 500 μm. The thickness of the second core 204at the central portion between each adjacent pair of the first cores 203is allowed to be slowly reduced from 4 μm at the connection area betweenthe slab waveguide 212 and the plurality of optical waveguides 213 to 0μm at the propagation length z of 500 μm. As a result, total excess losscorresponding to all 8-channel outputs can be reduced from 1.0 dB, whichis obtained in the case where the second core 204 is not provided, to0.2 dB. Further, the obtained PDL is as quite low as 0.05 dB or less.

[Third Embodiment]

An optical waveguide circuit according to a third embodiment of thepresent invention is shown in FIGS. 8 to 11. FIG. 8 is a top view; FIG.9 is a cross-sectional view taken along the line IIIb—IIIb in FIG. 8;FIG. 10 is a cross-sectional view taken along the line IIIc—IIIc in FIG.8; and FIG. 11 is a cross-sectional view taken along the line IIId—IIIdin FIG. 8.

The configuration of the optical waveguide circuit is obtained bymodifying the second embodiment. More specifically, 1-input and2-outputs circuit, that is, Y-branch type circuit is applied to theconfiguration of the second embodiment.

A concrete example of the third embodiment will be described below. Thematerial of the substrate, the material and size of each waveguidelayers, the refractive index and the like are the same as those in thefirst embodiment. Lower cladding 302 is formed on substrate 301. On thelower cladding 302, first cores 303 (303A, 303B, 303T), second core 304,and upper cladding 305 are sequentially formed. Waveguide 311 branchesinto two waveguides 313A and 313B through taper waveguide 312.

The interval between the first core 303A of the waveguide 313A and thefirst core 303B of the waveguide 313B at a connection point between twowaveguides 313A and 313B and taper waveguide 312 is set to 1 μm. Thefirst core 303T denotes the first core of the taper waveguide 312. Theinterval between the cores 303A and 303B that diverge from the core 303Tis allowed to spread up to 20 μm with the assumption that thepropagation length z from the connection point between the taperwaveguide 312 and two waveguides 313A and 313B is 300 μm. The thicknessof the second core 304 at the central portion between the first cores303 is allowed to be slowly reduced from 4 μm at the connection areabetween the taper waveguide 312 and two waveguides 313A and 313B to 0 μmat the propagation length z of 300 μm. As a result, total excess losscorresponding to 2-channel outputs can be reduced from 0.5 dB, which isobtained in the case where the second core 304 is not provided, to 0.1dB. Further, the obtained PDL is as quite low as 0.05 dB or less.

[Fourth Embodiment]

An optical waveguide circuit according to a fourth embodiment of thepresent invention is shown in FIGS. 12 to 15. FIG. 12 is a top view;FIG. 13 is a cross-sectional view taken along the line IVb—IVb in FIG.12; FIG. 14 is a cross-sectional view taken along the line IVc—IVc inFIG. 12; and FIG. 15 is a cross-sectional view taken along the lineIVd—IVd in FIG. 12.

The optical waveguide circuit is obtained by applying the configurationof the optical waveguide circuit of the second embodiment to an AWG andincludes a first slab waveguide 412 connected to one or more inputwaveguide 411, a second slab waveguide 414 connected to one or moreoutput waveguide 415, and arrayed waveguides 413 formed between thefirst and second slab waveguides with optical path length differences.

The arrayed waveguides 413 include lower cladding 402 formed onsubstrate 401, a plurality of first cores 403 formed on the lowercladding 402, second core 404 that covers the upper and both sidesurfaces of each of the first cores 403 and is formed between eachadjacent pair of first cores 403 at least at connection areas betweenthe first and second slab waveguides 412 and 414 and arrayed waveguides413 and at the portion near the connection areas, and upper cladding 405so formed on the lower cladding 402 as to bury the first and secondcores 403 and 404. The refractive index of the second core 404 is higherthan those of the claddings 402 and 405, and the boundaries between thesecond core 404 and claddings 402 and 405 are made smooth. The secondcore 404 is formed from the upper portion of a first core 403S of theslab waveguide 412 to the upper portions of the first cores 403 of thearrayed waveguides 413, and from the upper portion of the first core403S of the slab waveguide 414 to the upper portions of the first cores403 of the arrayed waveguides 413. The film thickness of the second core404 formed between the first cores 403 of the arrayed waveguides 413becomes thinner as each interval between the first cores 403 becomeswider. The formation of the second core 404 prevents the signal lightthat propagates through the first slab waveguide 412 and enters the gapsbetween the first cores 403 from being radiated into the claddings 402and 405, thereby reducing the radiation loss.

A concrete example of the fourth embodiment will be described below. AnAWG (channel interval: 100 GHz, number of channels: 4) is manufacturedwith parameters of the star-shaped branch circuit such as the materialof the substrate, the material and size of each waveguide layers, therefractive index or the like being the same as those in the secondembodiment. As a result, the AWG insertion loss can be reduced from 2.5dB, which is obtained in the case where the second core 404 is notprovided, to 1.0 dB. Further, the obtained PDL in the 1 nm transmissionwavelength band is as much low as 0.15 dB or less.

[Fifth Embodiment]

An optical waveguide circuit according to a fifth embodiment of thepresent invention is shown in FIGS. 16 to 18. FIG. 16 is a top view;FIG. 17 is a cross-sectional view taken along the line Vb—Vb in FIG. 16;and FIG. 18 is a cross-sectional view taken along the line Vc—Vc in FIG.16.

The optical waveguide circuit is a coupler including proximitywaveguides 512 having a plurality of first cores 503A and 503B that arenearby arranged to each other. The proximity waveguides 512 have acoupling length L. The coupler includes input waveguide 511, proximitywaveguides 512, and output waveguides 513A and 513B. The proximitywaveguides 512 includes lower cladding 502 formed on substrate 501, aplurality of first cores 503A and 503B formed in parallel to each otheron the lower cladding 502, second core 504 that covers at least a partof the first cores 503A and 503B respectively and is formed between thefirst cores 503A and 503B, and upper cladding 505 so formed on the lowercladding 502 as to bury the first cores 503A and 503B and second core504.

A further description will be given of the second core 504. The secondcore 504 is formed in the area where the proximity waveguides 512 areformed and in the vicinity of the area so as to cover the upper surfaceand both side surfaces of each of the first cores 503A and 503B.Particularly in the area where the proximity waveguides 512 are formed,in which the interval between the first cores 503A and 503B is narrow,the gap between the first cores 503A and 503B is filled with the secondcore 504. The film thickness of the second core 504 at the portionbetween the first cores 503A and 503B becomes thinner as the first cores503A and 503B get away from the formation area of the proximitywaveguides 512 and the interval between the first cores 503A and 503Baccordingly becomes wider. Note that it is only necessary for the filmthickness of the second core 504 to sufficiently fill up the gap betweenthe first cores in the formation area of the proximity waveguides 512.It is preferable that the refractive index of the second core 504 ishigher than that of the upper cladding 505 and is lower than those ofthe first cores 503A and 503B. The boundary between the second core 504and upper cladding 505 is made smooth.

The formation of the second core 504 can increase the equivalentrefractive index between the first cores 503A and 503B in the formationarea of the proximity waveguides 512 as compared to the area where theinterval between the first cores is sufficiently large, resulting inreduction of Δ. Therefore, it is possible to increase effusion of asignal light from the first cores 503A and 503B. As a result, even whenthe interval between the first cores 503A and 503B of the proximitywaveguides 512 is not changed from the conventional optical waveguidecircuit, it is possible to obtain an advantage corresponding to thatobtained by reducing an interval between the proximity waveguides 512,so that the coupling length L of the coupler can be shortened.

A concrete example of the fifth embodiment will be described below. Asilicon substrate is used as the substrate 501. A BPSG is used as thematerials of the lower and upper claddings 502 and 505. The filmthicknesses of the lower and upper claddings 502 and 505 are set to 7μm, and the refractive indexes thereof are set to 1.450. SiON is used asthe materials of the first cores 503A and 503B. Both the thicknesses andwidths of the first cores 503A and 503B are set to 2 μm. The refractiveindexes thereof are set to 1.526, with the result that the refractiveindex differences Δ between the first cores 503A and 503B and claddings502 and 505 are 5%, respectively. BPSG is used as the material of thesecond core 504. The film thickness of the second core 504 is set to 0.4μm, and the refractive index thereof is set to 1.511. A heat reflow isapplied to smooth the surface of the second core 504. The intervalbetween the cores 503A and 503B at the formation area of the proximitywaveguides 512 is set to 2 μm. As a result, the coupling length Lrequired for the waveguide circuit, in which a signal light inputthrough the input waveguide 511 is allowed to branch into the outputwaveguides 513A and 513B for output, is shortened from 1,500 μm, whichis obtained in the case where the second core 504 is not provided, to200 μm.

[Sixth Embodiment]

FIGS. 19 to 22 and FIGS. 23 to 26 are manufacturing process viewsshowing a manufacturing method of an optical waveguide circuit accordingto a sixth embodiment of the present invention. Lower cladding 602/702and a first core layer are formed on substrate 601/701. The first corelayer is selectively etched by photolithography and reactive ion etchingto form first core 603/703. After that, second core layer 604A/704A isso formed on the lower cladding 602/702 as to cover at least the uppersurface and both side surfaces of the first core 603/703 (FIG. 19 andFIG. 23).

The surface of the second core layer 604A/704A is made smooth by anapplication of heat-reflow process to form second core 604B/704B (FIG.20 and FIG. 24). In this time, the reflow process temperature and timeis controlled to allow the film thickness of the second core 604B/704Bat the position between the first cores to change into a desirable shapedepending on the interval between the adjacent first cores 603/703. Thatis, the film thickness of the second core 604B/704B is larger at theposition where the interval between the first cores is narrow. Contrary,the film thickness of the second core 604B/704B is smaller at theposition where the interval between the first cores is wide. In theabove configuration, the film thickness is allowed to change smoothly.

After that, the second core 604B/704B at the position sufficiently apartfrom the first core 603/703 is removed, as needed, by etching to obtainsecond core layer 604C/704C (FIG. 21 and FIG. 25).

Finally, upper cladding 605/705 is so formed on the lower cladding602/702 as to bury the first core 603/703 and second core 604C/704C tocomplete the optical waveguide circuit (FIG. 22 and FIG. 26).

The material having a refractive index higher than the refractiveindexes of the claddings 602 and 605/702 and 705 is used for the secondcore layer 604/704.

According to the experiment of the present inventor, a satisfactoryshape of the second core 604B/704B was obtained by forming a BPSG toobtain the second core layer 604A/704A and by applying a heat reflow tothe obtained second layer with a reflow process temperature of 850 to1,200° C. and reflow process time of 1 to 5 hours in a nitrogen, oxygen,or helium gas atmosphere under the condition that, for example, thethickness of the first core 603/703 is set to 1 to 8 μm and that theminimum interval between the first cores is set to 1 μm.

A chemical vapor deposition (CVD) method, flame hydrolysis deposition(FHD) method, sputtering method or the like can be applied to the filmformation of the upper and lower claddings 602, 605/702, 705, first core603/703, and second core layer 604A/704A.

According to the manufacturing method of the sixth embodiment, theoptical waveguide and optical waveguide circuit were manufactured usingthe materials and parameters described in the first to fifthembodiments. With this manufacturing method, the second core accordingto the first to fifth embodiments can be formed only through the filmformation process and reflow process that have been already establishedas well-known techniques. As a result, it is possible to manufacture alow-loss waveguide, a branch/converging circuit or AWG with low-loss andlow PDL, or a coupler with a short coupling length, in each of whichvariation of characteristics is minor in a wafer surface or betweenwafers and high manufacturing yield can be achieved.

INDUSTRIAL APPLICABILITY

As described above, the present invention relates to an opticalwaveguide circuit for use in optical communications and is effectivelyapplied to a branch circuit, converging circuit, coupler, or the like.

1. An optical waveguide circuit comprising: at least one opticalwaveguide; and a slab waveguide connected to said at least one opticalwaveguide, said at least one optical waveguide and slab waveguidecomprising: a first core; a cladding that buries said first core; and asecond core that is formed between said first core and cladding, whereinsaid second core is so formed throughout said at least optical waveguideand slab waveguide as to cover said first core, the refractive index ofsaid second core is higher than the refractive index of said cladding,and the boundary between said second core and cladding is made smooth.2. An optical waveguide circuit that allows an optical signalpropagating through at least one optical waveguide to branch into aplurality of optical waveguides, or converges optical signalspropagating through a plurality of optical waveguides into at least oneoptical waveguide, said at least one optical waveguide and plurality ofoptical waveguides comprising: a first core that branches from at leastone core to a plurality of cores or that is converged from a pluralityof cores into at least one core; a cladding that buries at least saidfirst core; and a second core formed between said first core andcladding, wherein each interval between the branches of said first corethat branches in the plurality of optical waveguides becomes wider assaid first core gets away from a branch point or converging point of anoptical signal, said second core of said plurality of optical waveguidesis formed in the gaps between said branches of first core at theposition in the vicinity of the branch point or converging point, saidsecond core is so formed throughout said at least one optical waveguideand plurality of optical waveguides as to cover said first core, therefractive index of said second core 1S higher than the refractive indexof said cladding, the boundary between said second core and cladding ismade smooth, and the film thickness of said second core formed in thegaps between said branches of first core becomes thinner as the intervalbetween said branches of first core becomes wider.
 3. The opticalwaveguide circuit according to claim 2 wherein said optical waveguidecircuit is a Y-shaped branch circuit.
 4. An optical waveguide circuitcomprising: a first slab waveguide connected at least one inputwaveguide; a second slab waveguide connected at least one outputwaveguide; and arrayed waveguides formed between said first and secondslab waveguides with optical path length differences, said first slabwaveguide, said second slab waveguide and said arrayed waveguidescomprising: a first core that branches in the arrayed waveguides andthat is converged into at least one core in said first or second slabwaveguides; a cladding that buries said first core: and a second coreformed between said first core and cladding, wherein said second core ofsaid arrayed waveguides is formed in the gaps between the branches ofsaid first core at connection areas between said first and second slabwaveguides and said arrayed waveguides and the portion near theconnection areas, said second core is so formed throughout said firstslab waveguide, arrayed waveguides, and second slab waveguide as tocover said first core, the refractive index of said second core ishigher than the refractive index of said cladding, the boundary betweensaid second core and cladding is made smooth, and the film thickness ofsaid second core formed in the gaps between said branches of first coreof said arrayed waveguides becomes thinner as the interval between saidbranches of first core becomes wider.
 5. An optical waveguide circuitcomprising proximity waveguides in which a plurality of first cores arenearby arranged to each other, said optical waveguide comprising: aplurality of first cores; a cladding that buries said first cores; and asecond core that is formed between said first cores and cladding tocover said first cores, wherein said second core is formed in the gapsbetween said first cores in said proximity waveguides, and said secondcore is not formed in the gaps between said first cores in thewaveguides other than said proximity waveguides, the refractive index ofsaid second core is higher than the refractive index of said cladding,and the boundary between said second core and cladding is made smooth.6. The optical waveguide circuit according to claim 1, wherein saidfirst core that is covered by said second core has a substantiallyrectangular cross-section, and said second core covers the upper surfaceand both side surfaces of said first core.
 7. The optical waveguidecircuit according to claim 2, wherein said first core that is covered bysaid second core has a substantially rectangular cross-section, and saidsecond core covers the upper surface and both side surfaces of saidfirst core.
 8. The optical waveguide circuit according to claim 4,wherein said first core that is covered by said second core has asubstantially rectangular cross-section, and said second core covers theupper surface and both side surfaces of said first core.
 9. The opticalwaveguide circuit according to claim 5, wherein said first core that iscovered by said second core has a Substantially rectangularcross-section, and said second core covers the upper surface and bothside surfaces of said first core.
 10. The optical waveguide circuitaccording to claim 1, wherein the thickness of said second core thatcovers at least a part of said first core is less than or equal to twicethe thickness of said first core.
 11. The optical waveguide circuitaccording to claim 2, wherein the thickness of the second core thatcovers at least a part of said first core is less than or equal to twicethe thickness of said first core.
 12. The optical waveguide circuitaccording to claim 4, wherein the thickness of said second core thatcovers at least a part of said first core is less than or equal to twicethe thickness of said first core.
 13. The optical waveguide circuitaccording to claim 5, wherein the thickness of said second core thatcovers at least a part of said first core is less than or equal to twicethe thickness of said first core.
 14. The optical waveguide circuitaccording to claim 1, wherein the refractive index of said second coreis less than or equal to 1.01 times the refractive index of said firstcore.
 15. The optical waveguide circuit according to claim 2, whereinthe refractive index of said second core is less than or equal to 1.01times the refractive index of said first core.
 16. The optical waveguidecircuit according to claim 4, wherein the refractive index of saidsecond core is less than or equal to 1.01 times the refractive index ofsaid first core.
 17. The optical waveguide circuit according to claim 5,wherein the refractive index of said second core is less than or equalto 1.01 times the refractive index of said first core.
 18. Amanufacturing method of an optical waveguide circuit comprising: atleast one optical waveguide; and a slab waveguide connected to said atleast one optical waveguide, said method comprising at least the stepsof: forming a core layer; selectively etching said core layer to form afirst core throughout said at least one optical waveguide and slabwaveguide; forming a second core layer that covers the upper surface andboth side surfaces of said first core, said second core layer being madeof a material having a refractive index higher than the refractive indexof said cladding; applying a heat reflow to said second core layer tosmooth the surface thereof to complete a second core; and forming saidcladding on said second core.
 19. A manufacturing method of an opticalwaveguide circuit that allows an optical signal propagating through atleast one optical waveguide to branch into a plurality of opticalwaveguides, or converges optical signals propagating through a pluralityof waveguides into at least one optical waveguide, said methodcomprising at least the steps of: forming a core layer; selectivelyetching said core layer to form a first core that branches from at leastone core to a plurality of cores or that is converged from a pluralityof cores into at least one core, each interval of the branches of saidfirst core becoming wider as said first core gets away from a branchpoint or converging point of an optical signal; forming a second corelayer on the upper portion of said first core and between the branchesof said first core throughout said at least one optical waveguide andplurality of optical waveguides, said second core layer being made of amaterial having a refractive index higher than the refractive index ofsaid cladding; applying a heat reflow to said second core layer tosmooth the surface thereof and forming a second core such that the filmthickness of said second core layer that is formed in the gaps betweenthe branches of said first core becomes thinner as the interval betweenthe branches of said first core becomes wider; and forming said claddingon said second core.
 20. A manufacturing method of an optical waveguidecircuit comprising: a first slab waveguide connected at least one inputwaveguide; a second slab waveguide connected at least one outputwaveguide; and arrayed waveguides including a plurality of cores andformed between said first and second slab waveguides with optical pathlength differences, said method comprising at least the steps of:forming a core layer; selectively etching said core layer to form saidfirst core that branches in connection points between said first andsecond slab waveguides and arrayed waveguides in said first slabwaveguide, arrayed waveguides, and second slab waveguide, each intervalof the branches of said first core becoming wider as said first coregets away from a connection point between said first and second slabwaveguides and the arrayed waveguides; forming a second core layer onthe upper portion of said first core and between the branches of saidfirst core at least throughout said first slab waveguide, arrayedwaveguides, and second slab waveguide, said second core layer being madeof a material having a refractive index higher than the refractive indexof said cladding; applying a heat reflow to said second core layer tosmooth the surface thereof and forming a second core such that the filmthickness of said second core layer that is formed in the gaps betweenthe branches of said first core becomes thinner as the interval betweenthe branches of said first core becomes wider; and forming said claddingon said second core.
 21. A manufacturing method of an optical waveguidecircuit comprising proximity waveguides in which a plurality of firstcores are nearby arranged to each other, said method comprising at leastthe steps of: forming a core layer: selectively etching said core layerto form the plurality of first cores; forming a second core layer on theupper portion of each of said first cores and between said first cores,said second core layer being made of a material having a refractiveindex higher than the refractive index of said cladding; applying a heatreflow to said second core layer to smooth the surface thereof to form asecond core in the gaps between said first cores in said proximitywaveguides such that said second core is not formed in the gaps betweensaid first cores in the waveguides other than said proximity waveguides;and forming said cladding on said second core.